In mobile radio, the use of heterodyne transmission architectures is increasingly being replaced by the use of homodyne architectures with direct conversion from baseband to a radio frequency.
If the baseband signal is in the form of a complex-value signal which has been split into an inphase component and a quadrature component which is orthogonal thereto, then a quadrature modulator or vector modulator is normally provided for converting the frequency of the modulation signal to a carrier frequency. A vector modulator normally comprises two Gilbert mixer cells. These up-convert the useful signal to a carrier frequency. In this case, a carrier signal generated by a frequency generator is supplied to one of the two radio-frequency mixers or multipliers in unaltered form and to the other with a phase shift of 90°. A summing element connected downstream of the two mixers combines the output signals with one another to form a transmission signal.
With a modulator of this kind, it is desirable to obtain sufficient carrier suppression and sideband suppression.
However, the carrier suppression is normally impaired by offset voltages, which may arise as a result of unavoidable component mismatches during mass production, for example. In addition, inadequate insulation between the carrier signal input and the signal output of the modulator may result in crosstalk by the carrier signal and hence in further impairment of the carrier suppression.
The ever greater bandwidth requirement, the increasing data rates, the demands for ever lower drawn current and the introduction of new modulation methods are continually increasing the demands on carrier suppression in a modulator. Taking the mobile radio standard GSM (Global System for Mobile communication) as a basis, the modulation method 8-PSK (Phase Shift Keying) provided in line with the GSM EDGE system increases the linearity demands and the demands on carrier suppression.
Inadequate sideband suppression may normally arise firstly from a discrepancy in the ideal phase difference of 90° for the carrier signals for the vector modulator. Secondly, any existing amplitude mismatch in the useful signal components may also result in impairment of the sideband suppression.
The document U.S. Pat. No. 4,243,955 specifies a method for improved carrier suppression in modulation systems. In this case, in addition to the actual carrier signal a quadrature signal is generated and is added to the modulated radio-frequency signal at the output of a modulator. Any carrier signal which shows through is suppressed on account of the opposing phases.
A further method for avoiding carrier crosstalk is specified in the document U.S. Pat. No. 5,574,994. In this document, a modulated, radio-frequency transmission signal is attenuated on the basis of the power gain in a feedback path, is down-converted to baseband, is amplified and is added to the inphase and quadrature paths in baseband as appropriate.
In addition, trimming methods during manufacture are also known, which involve spectrum analyzers being used to improve the carrier and sideband suppression by trimming the baseband signals. Furthermore, the offset voltages in the modulator circuits can be reduced by virtue of appropriate enlargement of the surface areas of the components used and by virtue of layout measures during circuit design.
It is also a known practice to measure, with a Schottky diode as a power detector, the radio-frequency modulation signal following amplification with a power amplifier. The resultant analog signal is then converted into a digital signal to ascertain the carrier suppression for trimming the baseband signals with a digital signal processor by means of computation.
The methods described have the common drawback that a relatively high level of circuit complexity is required in order to implement said methods in integrated circuits.